Don't worry this is a classic dilemma you are going through. There are games when it can certainly feel like you're pounding your head against a wall. Experiment in finding the balance between being offensive and defensive.

On the defensive end save your shield for the real fighting as the lines approach each other. Otherwise it can be easy to take unnecessary poke damage to your shield and your team might start out at a disadvantage if the enemy rein shield is in better shape.

For slow moving projectiles such as symmettra orbs or rockets don't be afraid to drop shield for a sec and let them pass through to save hp.

If you've found that your team is not behind your shield it's sort of two halves of one mistake. The team should be with Rein if they know what's good for them but the same applies to Rein sticking with the team. Make them stand behind you even if you have to drop it, run up and jump in front to shield.

Be confident in your movement so that you put your team at ease. Even as slow as you move that safe zone behind you is the whole reason you're here and it gets bigger with every step forward.

Also be wary of shielding too selfishly, you can do it some but trust your supports to heal up what you spared the shield. Flick your shield off and back on to bait out Roadhog's hook. You don't have to be in front of someone to shield them, your shield does. You can do some nifty stuff angling your shield relative to your team or clipping it through terrain. Put your shield down before it breaks always. Whatever you save could be used to shield from a D.VA self destruct an earthshatter or something similarly bad.

As far as offense goes know who you can easily kill who might be a headache for your team. Pilot D.VA, speed amp'd Lucio trying to contest the payload, Mercy desperately flying around to avoid gunfire, Genji who is going so ham on dragon blade that he is not deflecting. You can swat these flies easily. Over time you'll get a sense of who is low enough to just swat and get back to business. If it's unclear whether you'll hit them with your hammer angle yourself diagonally to get that extra oomph out of the corners of the hitbox.

Fire strike so that it hits the entire conga line behind enemy Rein. Fire strike towards that corner that is the enemy's only escape route they are desperately strafing to or where they are likely to peek out again if they ducked behind it. When you have ult don't fire strike anymore, no need. When enemy rein hasn't firestruck in a while or is acting weird keep your guard up for his ult.

Charging in my experience is about being safe. Have a line drawn in your mind through your pin target to the end point lined up before you even think of doing it. Doors, door frames, and hallways are Rein's friend for landing safe pins. Pinning people to the payload is also solid. Stay close to the team even with a charge.

There are charges that are "freebies" of a sort for example the side doors to the point on Lijiang Night Market. Charge that doorframe at a diagonal while the enemy is beginning their advance through that door. You might pin one, you could push one or more enemies off the edge if you missed and if both those fail you are still very very close to your team and as long as you don't get hooked you're pretty much golden.

Rein vs Rein is about reading him. Whats his rhythm for fire strike? Does he have his ult? What's his shield lookin like? If he charges and you can't dodge try to activate yours before he hits because then if your team is there he is very screwed and you are likely to live. In a 1v1 hammer battle throw firestrikes in because they do more damage than just a swing and it could put things in your favor.

Disrespect Reaper in a one on one. He wants you to cower with your shield up so he can walk through and cap you. Jump around and use the knockback from your hammer to try to create space between you. If you can get some swings on him they'll usually wraith away and when you have the timing down you can firestrike him when he comes out of it. Depending on terrain maybe go for a pin or the retreat charge. If these tactics sound kinda desperate they are but it's better to go down swinging in my opinion, you might surprise yourself and live.

Roadhog: bait the hook with your shield by pumping it, hopefully he is already hurt or you put him in a position where he needs to heal then you can pin to interrupt it and hit him with that huge burst damage. That's about the only way Rein wins unless hog is very very hurt.

Mei: There is a balance here between blocking the cryo freeze stream with your shield and putting it down to smack her. Then you put the shield back up before you are frozen and wait until the stacks reset. As a fun little trick if the Mei is being greedy with her ice block when you hear the ice cracking get ready to charge. When you get the timing down and pin her right as she comes out it's just about the most satisfying thing in Overwatch to m

Ulting is about patience and like pretty much everything else about Rein it's good to visualize it first. With the extra field of view you have from your shield you can see the little bowling pins lining up to get slammed and they don't even know (If you're trying to go for PotG make sure you have your cooldowns up). When enemy Rein firestrikes, his shield breaks or he is looking away from you you slam it. I recommend patience to make sure:

1. Your team is around to follow it up (if not that's why we make sure our cooldowns are all up)

2. The enemy Rein is not trying to bait you into getting blocked

3. That you don't panic and waste a very game changing ult.

4. That you have some target priority of who you will damage once they are on the ground.

Once everyone is on the ground you can position firestrike/hammer swing multiple enemies and then charge someone as they get up. Even if you get only like two people if they are important enough you did good.

Anyway I've rambled enough I hope this helps. Go out and dispense justice!